Method for producing laminate for configuring image display device, and image display device using the same

ABSTRACT

A method for producing a laminate for configuring an image display device is provided, capable of realizing simultaneously unevenness followability and anti-bubble reliability. A method for producing a laminate for configuring an image display device is proposed, comprising the following steps (1) and (2): 
     (1) The step of forming an adhesive composition into the form of a monolayer or a multilayer sheet, and by UV-crosslinking, primary-curing this thereby forming a pre-secondary curing transparent double-sided adhesive sheet. 
     (2) The step of layering two image display device-constituting members via the pre-secondary curing transparent double-sided adhesive sheet and then irradiating ultraviolet light from at least a first image display device-constituting member side, UV-crosslinking through this member and secondary-curing the pre-secondary curing transparent double-sided adhesive sheet.

TECHNICAL FIELD

The present invention relates to a method for producing a laminate forconfiguring an image display device, which can be used as a constitutingmember of an image display device such as a personal computer, a mobileterminal (PDA), a gaming machine, a television (TV), a car navigation, atouch panel or a pen tablet.

TECHNICAL BACKGROUND

In recent years, in order to improve the visibility of image displaydevices, filling of the gap between an image display panel such as LCD,PDP or EL and a protective panel or a touch panel member disposed on thefront side (viewing side) thereof has been carried out with apressure-sensitive adhesive sheet, adhesive or the like, therebysuppressing the diffused reflections that occur when there is nofilling, i.e., the diffused reflections at the air/layer interface, ofan incoming beam or an outgoing light from the displayed image.

For instance, a display device is described in Patent Reference 1, inwhich the gap between the LCD (liquid crystal panel display) and a touchpanel is filled with a transparent resin (adhesive), allowing thereflection of light to be reduced between the touch panel and the liquidcrystal display panel.

A problem has been pointed out, that when the gap between an imagedisplay panel and a protective panel or a touch panel member is filledwith a transparent resin (adhesive) in this way, if a defect or the likeexists at the interface between the image display panel and thetransparent resin (adhesive) or at the interface between the protectivepanel or the like and the transparent resin (adhesive), a gas build-upoccurs with this defect as the origin, by exposure under a hightemperature and high humidity environment or by exposure to a suddenchange in temperature, provoking bubbling or peeling.

Consequently, a variety of studies have been carried out in order tosolve such problems.

For instance, production methods for display devices with an opticalfunction member built-in are proposed in Patent References 2 and 3,which involves photo-curing (or heat-curing) an adhesive compositionbeforehand and producing an adhesive composition that is in a partiallycrosslinked state, then followed by bonding together an optical functionmember and a display portion via this adhesive composition, andthereafter heat-curing (or photo-curing) the adhesive composition byheat treatment (or light irradiation).

A method described in Patent Reference 4 involves including a highmolecular weight acrylic polymer within a pressure-sensitive adhesivecomposition thereby increasing cohesive strength, imparting resistanceto bubbling, and suppressing peeling.

In addition, the present applicants have proposed in Patent Reference 5or the like, a novel transparent adhesive sheet that can be adheredwithout generating residual air bubbles, even if there were anunevenness of on the order of 5 μm to 20 μm on the adhered surface, andfurthermore can be adhered without bubbling under a high temperatureenvironment of, for instance, on the order of 80° C. even if theadherend were a material that generates outgas such as plastic.

PRIOR ART REFERENCES [Patent References]

[Patent Reference 1] Japanese Patent Application Laid-open No.2004-077887

[Patent Reference 2] Japanese Patent Application Laid-open No.2009-109532

[Patent Reference 3] Japanese Patent Application Laid-open No.2009-109533

[Patent Reference 4] Japanese Patent Application Laid-open No.2004-210866

[Patent Reference 5] International Publication No. 2010/044229

SUMMARY OF THE INVENTION

[Problems to be Solved by the invention]

With the methods described in Patent References 2 and 3 mentioned above,a material that possesses both photo-curability and heat-curability isused, and since there is the risk that the organic peroxide that will beadded not only will cause gelation of the composition at processing butalso provoke such problems as bubbling or oxidation corrosion of theadherend due to reaction decomposition products of the organic peroxide,giving photo-curability and heat-curability to an adhesive sheet used insuch type of applications should be avoided.

In addition, although anti-bubble reliability becomes sufficient ifadhesive strength and cohesive strength are provided to an extent thatcan counter the gas pressure of an outgas generated from a protectivepanel or the like as described above, since this means the adhesivesheet is stiff by as much, if there were unevenness on the surface ofthe adherend or if a foreign substance or the like is present at thebonding interface, such unevenness cannot be followed sufficiently.

Thus, an object of the present invention is to provide a method forproducing a laminate for configuring an image display device capable ofsimultaneously realizing unevenness followability and anti-bubblereliability.

[Means to Solve the Problems]

The present invention proposes a method for producing a laminate forconfiguring an image display device having a constitution in which imagedisplay device-constituting members are layered via a transparentdouble-sided adhesive sheet, and the method for producing a laminate forconfiguring an image display device comprises at least the followingsteps in (1) and (2):

(1) the step of forming an adhesive composition into the form of amonolayer or a multilayer sheet, and primary-curing this to form apre-secondary curing transparent double-sided adhesive sheet byUV-crosslinking; and

(2) the step of layering two image display device-constituting membersvia the pre-secondary curing transparent double-sided adhesive sheet andthen irradiating ultraviolet light from at least one side of imagedisplay device-constituting member side, UV-crosslinking through thismember to secondary-cure the pre-secondary curing transparentdouble-sided adhesive sheet.

Since the pre-secondary curing transparent double-sided adhesive sheetobtained in the step (1) still has room for being cured further, inother words, still has room for being UV-crosslinked further and isflexible as much, even if there were unevenness on the surface of anadherend or if a foreign substance or the like were present at thebonding interface, it can follow these unevennesses sufficiently toadapt. Thus, in the step (2), if two image display device-constitutingmembers are layered via such a pre-secondary curing transparentdouble-sided adhesive sheet, each image display device-constitutingmember can be brought into tight contact adequately.

Then, in the step (2), by irradiating UV light from at least one side ofimage display device-constituting member side, UV-crosslinking throughthis member and secondary-curing, the pre-secondary curing transparentdouble-sided adhesive sheet can be crosslinked firmly, allowing for theprovision of adhesive strength and cohesive strength to an extent thatcan counter, for instance, the gas pressure of an outgas generated froma protective panel or the like.

In this way, method for producing a laminate for configuring an imagedisplay device of the present invention can realize simultaneouslyunevenness followability and anti-bubble reliability, which are ingeneral in a trade-off relationship.

MODES FOR CARRYING OUT THE INVENTION

Hereafter, an example of embodiment of the present invention will bedescribed; however, the present invention is not limited to theembodiment described below.

Method for Producing a Laminate for Configuring an Image Display Device>

The method for producing a laminate for configuring an image displaydevice according to the present embodiment (hereafter abbreviated as“the present production method”) is a method for producing a laminatefor configuring an image display device provided with a constitution inwhich image display device-constituting members are layered via atransparent double-sided adhesive sheet (hereafter abbreviated as “thepresent adhesive sheet”), the method for producing a laminate forconfiguring an image display device (hereafter abbreviated as “thepresent laminate”) comprising at least the following steps in (1) and(2):

(1) the step of forming an adhesive composition into the form of amonolayer or a multilayer sheet, and by UV-crosslinking, primary-curingthis thereby forming a pre-secondary curing transparent double-sidedadhesive sheet; and

(2) the step of layering two image display device-constituting membersvia the pre-secondary curing transparent double-sided adhesive sheet andthen irradiating UV light from at least a first image displaydevice-constituting member side, UV-crosslinking through this member andsecondary-curing the pre-secondary curing transparent double-sidedadhesive sheet.

With the methods described in prior art, an adhesive material thatpossesses both photo-curability and heat-curability is used to carry outprimary curing and secondary curing, which have different crosslinkingmeans; however, there is the risk that a heat-curing agent added forheat-curing purposes such as an organic peroxide, an isocyanatecompound, an epoxide compound or an amine compound not only will causegelation of the composition at processing but also provoke such problemsas bubbling or oxidation corrosion of the adherend due to reactiondecomposition products. In contrast, with the present production method,since primary curing and secondary curing are both carried out byUV-crosslinking, such matters of concern pertaining to heat-curingprocess can be eliminated.

Since the pre-secondary curing transparent double-sided adhesive sheetobtained in the step (1) still has room for being UV-crosslinked furtherand is flexible as much, even if there were unevenness on the surface ofan adherend or if a foreign substance or the like were present at thebonding interface, it can sufficiently follow, adapt to, wet andadequately be brought into tight contact with, these unevennesses. Then,in the step (2), by being secondary-cured, it can be crosslinked firmly,allowing for the provision of adhesive strength and cohesive strength toan extent that can counter for instance the gas pressure of an outgasgenerated from a protective panel or the like.

<Adhesive Composition>

First, in the present production method, an example of adhesivecomposition that can be used suitably for forming a monolayeredtransparent double-sided adhesive sheet (hereafter abbreviated as “thepresent adhesive composition”) will be described. However, this ismerely an example and the present production method is not limitedthereto.

With the present production method, it is important that, during thefirst ultraviolet irradiation, the adhesive composition isUV-crosslinked so as to have potential UV-reactivity, in other words, soas to leave UV-reactivity.

In addition, adhesive composition is a solvent-free system that does notcontain a solvent, and forming an adhesive composition into a film byhot-melting are desirable.

As a mean that is for UV-crosslinking the adhesive composition so as tobeing hot-melt forming capable and to leave UV-reactivity in this way,for instance, means in which a base polymer having a predetermined rangeof molecular weight is used, and, an intermolecular hydrogen-abstractingphoto-polymerization initiator is used as a crosslinking initiator, and,the amount of crosslinking agent is reduced, can be cited. However,there is no limitation to such means.

(Base Polymer)

From such points of view as adhesiveness, transparency and resistance toclimate, the present adhesive composition preferably uses a(meth)acrylic acid ester series polymer (hereafter abbreviated as“acrylic acid ester series (co-)polymer” with the meaning of includingcopolymers) as the base resin.

The acrylic acid ester series (co-)polymer serving as the base resin canbe prepared by suitably selecting the types and the composition ratiosof the acrylic monomer or the methacrylic monomer, and further, thepolymerization conditions, which are used for polymerizing this, therebysuitably adjusting physical characteristics such as the glass transitiontemperature (Tg) and molecular weight.

As acrylic monomers or methacrylic monomers that are used forpolymerizing an acrylic acid ester (co-)polymer, for instance,2-ethylhexyl acrylate, n-octyl acrylate, isooctyl acrylate, n-butylacrylate, ethyl acrylate, methyl methacrylate, methyl acrylate, and thelike, can be cited. These that possess a hydrophilic group, an organicfunctional group or the like, including vinyl acetate, hydroxyethylacrylate, acrylic acid, glycidyl acrylate, acrylamide, acrylonitrile,methacrylonitrile, fluorine acrylate, silicon acrylate and the like, canalso be used.

As polymerization treatments that use these monomers, well-knownpolymerization methods such as solution polymerization, emulsionpolymerization, block polymerization and suspension polymerization canbe adopted, in which case, an acrylic acid ester copolymer can beobtained by using a polymerization initiator such as aheat-polymerization initiator or a photo-polymerization initiatoraccording to the polymerization method.

Preferably, the present adhesive composition is a solvent-free system,that is to say, does not contain a solvent, is hot-melt formable, and atthe primary cured stage, has adequate adhesive strength and hasflexibility that enables following the unevenness or a foreign substanceon an adherend surface. Film production by hot-melting is not possiblewhether the molecular weight of the base polymer is too large or toosmall. In addition, if the molecular weight of the base polymer is toosmall, there is the possibility that even if primary cured, no adhesivestrength is exerted, or, being too soft, handleability is poor;conversely, if the molecular weight is too large, being hard at theprimary cured stage, there is the possibility of not being able tofollow the unevenness or a foreign substance on an adherend surface.

Thus, from such point of view, it is desirable to use an acrylic acidester series (co-)polymer in which the mass average molecular weight ofthe base polymer is 100,000 to 700,000, in particular 200,000 orgreater, or 600,000 or lower, and above all, in particular 250,000 orgreater or 500,000 or lower.

Among these, using an acrylic acid ester series (co-)polymer which massaverage molecular weight (MW)/number average molecular weight (MN) is 5to 10, of which 6 or greater or 9 or less, is more desirable. A largemass average molecular weight/number average molecular weight means thatthe molecular weight distribution is broad, and if this value is aslarge as on the order of 5 to 10, since the low molecular weightcomponents and the high molecular weight components respectivelycontribute to capabilities on par with their molecular weights, such asfluidity, wettability and cohesive strength, there is a tendency forprocessability and adhesive capability to be better than those withnarrow (homogeneous) molecular weight distributions.

In addition, among the acrylic acid ester series (co-)polymers, it isdesirable to use acrylic acid ester random copolymers, and among these,acrylic acid ester random copolymers containing two species of monomerwith large differences between the glass transition temperature (Tg) ofeach of the monomer component constituting the random copolymer, that isto say, for each monomer component constituting the acrylic ester randomcopolymer, the glass transition temperature (Tg) of the polymerpolymerized with only a single monomer each.

In so doing, the difference in glass transition temperature (Tg) betweenthe two species of monomer component is preferably 25 to 300° C., inparticular 40° C. or higher or 200° C. or lower, of which in particular60° C. or higher or 180° C. or lower, and further, all the morepreferably 100° C. or higher or 180° C. or lower.

Concretely, it is desirable that the glass transition temperature (Tg)of the first monomer component is −100 to 0° C., in particular −80 to−20° C., and the glass transition temperature (Tg) of the second monomercomponent is 0 to 250° C., in particular 20 to 180° C.

As the monomer component with the lower glass transition temperature(Tg), for instance, a monomer component with a Tg of −100 to 0° C., itis desirable to use an acrylic acid ester in which the number of carbonsof the side chain is 2 or greater, and in particular 4 or greater.

Meanwhile, as the monomer component with high glass transitiontemperature (Tg), for instance, a monomer component with a Tg of 0 to250° C., it is desirable to use a vinyl monomer or a (meth)acrylicmonomer, or the like, having the side chain of a hydrocarbon in whichthe number of carbons is 2 or lower, in addition a cyclic structure suchas an alicyclic structure, a heterocycle or an aromatic, or a functionalgroup such as a carboxyl group, a hydroxyl group, an amino group, anamide group, a glycidyl group, an acetyl group or an isocyanate group.

The indentation hardness of the transparent double-sided adhesive sheetcan be adjusted by adjusting the proportions of the monomer componentwith high Tg (i.e., the monomer for which the glass transitiontemperature when the monomer has been polymerized alone is high) and themonomer component with low Tg (i.e., the monomer for which the glasstransition temperature when the monomer has been polymerized alone islow) in the acrylic acid ester series copolymer serving as the basepolymer. For instance, in order to raise the indentation hardness, itsuffices to increase the contents in the monomer component with high Tg.Conversely, when rendering the sheet flexible, it suffices to increasethe contents in the monomer component with low Tg.

In addition, it is possible to lower the hardness by softening with aplasticizer, or adjust the hardness suitably with additives such asoligomers.

(Crosslinking Agent)

As crosslinking monomers (crosslinking agents) used when crosslinking anacrylic acid ester series (co-)polymer, for instance, multi-functional(meth)acrylates having two or more (meth)acryloyl groups are desirable.

Regarding the amount of crosslinking agent, since, if the amount ofcrosslinking agent is large, the reaction proceeds rapidly andregulation of the reaction is difficult, it is desirable to adjust theamount of crosslinking agent so as to be able to stop crosslinking evenin mid-course.

From such point of view, the amount of crosslinking agent, with respectto 100 parts by mass of base polymer, is preferably 0 to 30 parts bymass, in particular 20 parts by mass or less, of which 10 parts by massor less, and among these in particular 5 parts by mass or less.

A reason for being able to reduce the amount of crosslinking agent inthis way is also that an intermolecular hydrogen-abstractingphoto-polymerizing initiator is used as crosslinking initiator.

(Crosslinking Initiator)

It is important to adopt an intermolecular hydrogen-abstractingphoto-polymerizing initiator (also referred to as “hydrogen-abstractingphoto-initiator”) as the crosslinking initiator used in the presentadhesive composition.

A photo-polymerization initiator generates a radical by lightirradiation and becomes the starting point of the polymerizationreaction in the system, with the generation mechanisms for(meth)acryloyl group and vinyl group reactive radicals being dividedbroadly into two classes: the intramolecular cleavage type, in whichradicals are generated by cleavage and decomposition of an initiator'sown single bond, and the hydrogen-abstraction type, in which radicalsare generated by exciting a hydrogen from a hydroxyl group, or the like,in the system.

Those of the intramolecular cleavage type decompose and become differentcompounds when generating radicals by light irradiation, such that oncereacted, they no longer have functions as reaction initiators, whilethose of the hydrogen-abstraction type revert to the original compoundseven after inactivation and can be used repeatedly as reactioninitiators by light irradiation. Consequently, in order to conferpotential photo-curability, it is important to use aphoto-polymerization initiator of the hydrogen-abstraction type.

As hydrogen-abstracting photo-initiators, for instance, any amongbenzophenone, Michler ketone, dibenzosuberone, 2-ethyl anthraquinone,thioxanthone, benzyl and the like, or derivatives thereof, or mixedcomponents from the combination of two or more species thereof can beused. However, there is no limitation to the substances given above asphoto-initiators of the hydrogen-abstraction type. In addition,photo-polymerization initiators of the intramolecular cleavage type maybe used in combination in a variety of proportions if those of thehydrogen-abstraction type are contained in the adhesive sheet.

The amount of photo-polymerization initiator added is not limited inparticular, and in general is adjusted preferably in a proportion of 0.1to 10 parts by mass, in particular 0.2 parts by mass or more or 5 partsby mass or less, and among these, 0.5 parts by mass or more or 3 partsby mass or less, with respect to 100 parts by mass of base resin.However, this range may be exceeded in balancing with other elements.

(Other)

When crosslinking an acrylic acid ester series (co-)polymer, asnecessary a reaction catalyst (tertiary amine series compound,quaternary ammonium series compound, tin laurate compound, and the like)may be added suitably.

(Multilayer Constitution)

When the present adhesive sheet is to be made into a multilayeredtransparent double-sided adhesive sheet, that is to say, when forming atransparent double-sided adhesive sheet with a layered constitutionprovided with an intermediate layer and outermost layers, it suffices toform the outermost layers thereof from the present adhesive compositiondescribed above.

When a layered constitution comprising an intermediate layer andoutermost layers is to be given, the present adhesive sheet ispreferably formed using the present adhesive composition describedabove, since it is desirable that the outermost layer is provided withunevenness followability and anti-bubble reliability as in the case ofthe monolayer described above. Meanwhile, it is desirable that theintermediate layer has light permeability to an extent that does notinhibit secondary curing reaction of outermost layers and has propertiesthat elevate cuttability and handleability without loosing transparency,since it does not contribute to the bonding of image displaydevice-constituting members.

While there is no particular limitation if the species of the basepolymer forming the intermediate layer is a transparent resin, using asimilar acrylic acid ester series (co-)polymer as described above isdesirable. In so doing, from such points of view as securingtransparency and ease of production, using an identical resin to thebase polymer of the outermost layer is desirable.

The intermediate layer may be formed so as to be cured byUV-crosslinking, or may be formed so as to be cured by heat. Inaddition, it may be formed so as not to be post-cured in particular.However, when tightness of contact with the outermost layers or the likeis considered, forming so that it post-cures is desirable, and inparticular, forming so that it UV-crosslinks is desirable.

In so doing, since light transmittance drops if the content incrosslinking initiator becomes large, it is desirable that theUV-crosslinking agent is contained at a lower content percentage thanthe content percentage in the outer layer of the crosslinking initiatorin the intermediate layer.

(Thickness)

From the points of view of unevenness followability and thinning soughtin the application members, the thickness of the present adhesive sheetis preferably 50 μm to 1 mm, and in particular 100 μm or greater or 500μm or less is desirable.

In addition, when the sheet is to have a multilayer constitution, theratio between the thickness of each of the outermost layers and thethickness of the intermediate layer is preferably 1:1 to 1:20, of which1:2 to 1:10 is further desirable.

If the thickness of the intermediate layer is in the range describedabove, it is desirable, as the contribution of the thickness of theadhesive layer in the laminate does not become excessively large, andworkability pertaining to cutting and handling does not degrade due tobeing excessively soft.

In addition, if the outermost layer is in the range described above, itis desirable, as followability of unevenness and curved surface does notdegrade, and adherent strength and wettability towards the adherend canbe maintained.

<Step (1)>

In Step (1), primary curing by UV-crosslinking is important so that thetransparent double-sided adhesive sheet has potential UV-reactivity, inother words, so as to leave UV-reactivity.

In Step (1), it suffices to obtain a pre-secondary curing transparentdouble-sided adhesive sheet, for instance, by heat-melting (hot-melting)the present adhesive composition, coating this over a transparentrelease resin sheet to form a sheet of single layer or multiple layers,then, irradiating UV light through the transparent release resin sheetto UV-crosslink the present adhesive composition.

In so doing, while it is possible to adjust the extent ofUV-crosslinking by controlling the amount of UV light irradiated, it isalso possible to irradiate UV light through the transparent releasesheet as described above thereby partially blocking the UV light so asto adjust the extent of UV-crosslinking.

Here, as examples of transparent release sheet, for instance, thosecomprising a cast film or a stretched film of the polyester series, thepolypropylene series or the polyethylene series, release-treated bybeing coated with a silicon resin, release papers or the like, can besuitably selected and used, and in particular, release films withdifferent peel strengths and release films with different thicknessescan be used.

The indentation hardness (C2 Asker hardness) (a) of the pre-secondarycuring transparent double-sided adhesive sheet obtained in Step (1) ispreferably in the range of 10≦(a)<50.

This C2 Asker hardness (a) is a value that may be measured with an Askerdurometer (Asker C2L) by sequentially layering transparent double-sidedadhesive sheets so that the total thickness is within the range of 5 mmto 7 mm.

The transparent double-sided adhesive sheet prior to adhesion to membersrequires a suitable degree of resilience to be rolled-up into rolls orcut, and also requires adhesive strength and wettability for bondingwith members. Therefore, if the C2 Asker hardness (a) is 10 or greater,there is no risk that the sheet is too soft and lacks workabilitypertaining to rolling, and if the C2 Asker hardness (a) is less than 50,even if there were unevenness on the surface of an adherend or if aforeign substance or the like were present at the bonding interface, thesheet can sufficiently follow, adapt to, wet and adequately be broughtinto tight contact with, these unevennesses. Thus, having the C2 Askerhardness (a) in the range described above allows workability andadhesive physical properties to be compatible, and in addition, allowsfollowability of steps due to printed portions or the like to besecured.

From such points, a C2 Asker hardness (a) of 15 or greater or 45 or lessis all the more desirable, of which 20 or greater or less than 33 isfurther desirable.

The thickness of the transparent double-sided adhesive sheet, the amountof UV light irradiated, the radiation wavelength, the radiationapparatus, and the like, can be suitably adjusted.

<Step (2)>

In Step (2), layering two image display device-constituting members viathe pre-secondary curing transparent double-sided adhesive sheetobtained in Step (1) and then irradiating UV light from at least thefirst image display device-constituting member side, UV-crosslinkingthrough this member and secondary-curing the pre-secondary curingtransparent double-sided adhesive sheet, is important

Here, for instance touch panels, image display panels, surfaceprotection panels, polarization films, and the like, can be cited as theimage display device-constituting members described above, which may beany among these or laminates comprising combinations of two or morespecies.

Since a sufficient amount of light at a wavelength that is effective forexciting a photo-initiator within the transparent double-sided adhesivesheet causing it to generate a radical needs to arrive in order toprovoke UV-crosslinking reaction in the transparent double-sidedadhesive sheet via an image display device-constituting member, it isdesirable that the UV transmittance of one of the image displaydevice-constituting members is at a given level or higher.

In particular, for instance, when a glass plate is to be layered on theside of the pre-secondary curing transparent double-sided adhesive sheetwhere UV light is to be irradiated, the UV transmittance of the glassplate is preferably at a given level or higher, and, for instance, whena glass plate, an adhesive and a protection sheet are to be layered, theUV transmittance of the laminate of the glass plate, adhesive andprotection sheet is preferably at a given level or higher.

Thus, the UV transmittance, that is to say, the light transmittance inthe 315 nm to 400 nm wavelength range of UV-A waves, of the imagedisplay device-constituting member layered on the ultravioletirradiation side of the pre-secondary curing transparent double-sidedadhesive sheet is preferably 20% or greater, and in particular 30% orgreater, of which in particular 40% or greater, is all the moredesirable.

As members that may have such a light transmittance, those constitutedfrom, for instance, a polycarbonate resin, an acrylic resin, a polyvinylchloride resin, a polyester resin, a cellulose resin such as triacetylcellulose or diacetyl cellulose, a styrene resin, and the like, can becited. Among these, polycarbonate resins, acrylic resin, triacetylcellulose resins and the like, have high hygroscopicity, and have theproperty that the moisture absorbed by the resin during storage is proneto being released as outgas from the resin when used under hightemperature. According to the method for producing a laminate forconfiguring an image display device of the present invention, sincebubbling caused by such outgas releases can be suppressed, resin membersconstituted from the polycarbonate resin, acrylic resin and triacetylcellulose resin and the like, can be used as resin members constitutingthe laminate.

The indentation hardness (C2 Asker hardness) (b) of the post-secondarycuring transparent double-sided adhesive sheet obtained in Step (2) ispreferably in the range of 33≦(b)≦80.

This C2 Asker hardness (b) is a value that may be measured with an Askerdurometer (Asker C2L) by sequentially layering transparent double-sidedadhesive sheets so that the total thickness is within the range of 5 mmto 7 mm.

By setting the C2 Asker hardness (b) of the post-secondary curingtransparent double-sided adhesive sheet to 33 or greater, the sheet hassufficient adhesive strength and cohesive strength, and can counter forinstance the gas pressure of an outgas generated from a protective panelor the like. Meanwhile, by setting the C2 Asker hardness (b) to 80 orlower, it is possible to eliminate such concerns as, the sheet being toostiff, the function as pressure-sensitive adherent is lost with aninferior tight contact strength, causing peeling and lifting to occur.

By adjusting the C2 Asker hardness (b) of the post-secondary curingtransparent double-sided adhesive sheet to the range described above inthis way, allows the provision of a cohesive strength that can resistthe gas pressure that gradually aggregates after bonding at the bondinginterface during long-term storage or under high temperature and highhumidity environment, and a tight contact strength of suitable degree toan extent that the sheet does not peel.

From such point of view, it is further desirable that the C2 Askerhardness (b) of the post-secondary curing transparent double-sidedadhesive sheet is 35 or greater or 60 or lower, of which 38 or greateror 50 or lower is all the more desirable.

In terms of the relationship between the C2 Asker hardness (a) of thepre-secondary curing transparent double-sided adhesive sheet and the C2Asker hardness (b) of the transparent double-sided adhesive sheet aftersecondary curing, it is desirable to control the hardnesses so as tohave a difference of two points or greater, that is to say, to berelated by (b)−(a)≧2, of which (b)−(a)≧3 and among which (b)−(a)≧4 areall the more desirable.

<Laminate Used in Constituting an Image Display Device>

As laminates used in constituting an image display device that may beproduced by the present production method, constitutive members of imagedisplay devices such as LCDs, PDPs or ELs such as for, for instance,personal computers, mobile terminals (PDAs), gaming machines,televisions (TVs), car navigations, touch panels, pen tablets, and thelike, can be cited.

To cite one concrete example, in the image display device of a cellularphone, sometimes a polarization film, or the like, is layered over aliquid crystal panel display (LCD), and above this, a plastic protectivepanel is layered via an adhesive or a sheet. In so doing, a PVA(polyvinyl alcohol) or triacetyl cellulose resin is sometimes used asthe constitutive material of the polarization film, which have beendetermined to be prone to release outgas.

Thus, if the present laminate is used as a laminate comprising theconstitution of protective panel/adhesive sheet/polarization film, whenused under high temperature, bubbling due to outgas released from theprotective panel or the polarization film can be suppressed effectively.

Elsewhere, constitutions such as, for instance, liquid crystalpanel/adhesive sheet/touch panel, liquid crystal panel/adhesivesheet/protective panel, liquid crystal panel/adhesive sheet/touchpanel/adhesive sheet/protective panel, polarization film/adhesivesheet/touch panel and polarization film/adhesive sheet/touchpanel/adhesive sheet/protective panel can be cited as constitutionexamples of the present laminate.

(Explanation of the Expressions and the Like)

In general, “sheet”, by definition under JIS, refers to a product thatis thin and flat, of which the thickness is small compared to the lengthand width; and in general, “film” refers to a thin and flat product, ofwhich the thickness is extremely small compared to the length and width,maximum thickness is arbitrarily limited, and provided in general in theform of a roll (Japanese Industrial Standard JIS K6900). However, theboundary between a sheet and a film is not certain, and since there isno need in the present invention to discriminate the two in wording, inthe present invention, “sheet” is deemed included even when referring to“film”, and “film” is deemed included even when referring to “sheet”.

In addition, when the expression “panel” is used, such as in imagedisplay panel and protective panel, this includes plate, sheet and film.

In the present Specification, when the statement “X to Y” (X and Y areany numbers) is made, unless expressly stated otherwise, along with themeaning of “X or greater but Y or less”, the meaning of “preferablylarger than X” or “preferably smaller than Y” are also included.

In addition, when the statement “X or greater” (X is any number) ismade, unless expressly stated otherwise, the meaning of “preferablylarger than X” is also included, and when the statement “Y or less” (Yis any number) is made, unless expressly stated otherwise, the meaningof “preferably smaller than Y” is also included.

EXAMPLES

Hereafter, the present invention will be explained in further detail byway of examples and Comparative Examples. However the present inventionis not limited to these.

(Intermediate Resin Layer-Forming Layered Sheet 1)

An acrylic acid ester copolymer A (Mw=440,000; Mn=62,000; Mw/Mn=8;theoretical Tg: −50° C.) was prepared, comprising 75 parts by mass of2-ethylhexyl acrylate (homopolymer Tg: -70° C.), 20 parts by mass ofvinyl acetate (homopolymer Tg: +32° C.) and 5 parts by mass of acrylicacid (homopolymer Tg: +106° C.), which were random-copolymerized.

100 g of the UV-curing resin propoxylated pentaerythritol triacrylate(manufactured by Shin-Nakamura Chemical Co., Ltd., “ATM-4PL”) as acrosslinking agent and 15 g of 4-methyl benzophenone as aphoto-polymerization initiator were mixed to 1 kg of this acrylic acidester copolymer A to prepare an intermediate resin layer composition(A-1).

The intermediate resin layer composition (A-1) was heat-melted andcoated with an applicator so as to become 110 μm in thickness onto arelease-treated polyethylene terephthalate film (manufactured by PanacCorporation, “NP75Z01”; thickness: 75 μm) serving as a coating substrateand then covered with a release-treated polyethylene terephthalate film(manufactured by Toyobo Co., Ltd., “E7006”; thickness: 38 μm) to producean intermediate resin layer-forming layered sheet 1 comprising PETfilm/UV-crosslinkable intermediate resin layer (A-1; thickness: 110μm)/PET film.

(Intermediate Resin Layer-Forming Layered Sheet 2)

An intermediate resin layer composition (A-2) was prepared in a similarmanner to the intermediate resin layer-forming layered sheet 1 describedabove except that 200 g of trimethylolpropane triacrylate was mixed asthe crosslinking agent and 50 g of 1-hydroxy-cyclohexylphenyl ketone wasmixed as the photo-polymerization initiator.

The intermediate resin layer composition (A-2) was heat-melted andcoated with an applicator so as to become 130 82 m in thickness onto arelease-treated polyethylene terephthalate film (manufactured by PanacCorporation, “NP75Z01”; thickness: 75 μm) serving as a coating substrateand then covered with a release-treated polyethylene terephthalate film(manufactured by Toyobo Co., Ltd., E7006; thickness: 38 μm) to producean intermediate resin layer-forming layered sheet 2 comprising PETfilm/UV-crosslinkable intermediate resin layer (A-2; thickness: 130μm)/PET film.

(Pressure-Sensitive Adhesive Layer-Forming Layered Sheet 1)

Added and mixed into 1 kg of acrylic acid ester copolymer A was 20 g of4-methyl benzophenone as photo-polymerization initiator to prepare apressure-sensitive adhesive composition (B-1).

This pressure-sensitive adhesive composition (B-1) was heat-melted,coated and formed into a sheet so that the thickness was 35 μm over arelease-treated polyethylene terephthalate film (manufactured byMitsubishi Plastics, Inc., “MRA75”; thickness: 75 μm) serving as acoating substrate and covered with a polyethylene terephthalate film(manufactured by Toyobo Co., Ltd., “E7006”; thickness: 38 μm) that hadbeen subjected to release treatment to produce a pressure-sensitiveadhesive layer-forming layered sheet 1 comprising PETfilm/UV-crosslinkable pressure-sensitive adhesive layer (B-1; thickness:35 μm)/PET film.

(Pressure-Sensitive Adhesive Layer-Forming Layered Sheet 1′)

A pressure-sensitive adhesive layer-forming layered sheet 1′ comprisingPET film/UV-crosslinkable pressure-sensitive adhesive layer (B-1′;thickness: 35 μm)/PET film was produced in a similar manner to thepressure-sensitive adhesive layer-forming layered sheet 1 describedabove, except that the coating substrate was changed to arelease-treated polyethylene terephthalate film (manufactured byMitsubishi Plastics, “MRF50”; thickness: 50 μm).

(Pressure-Sensitive Adhesive Layer-Forming Layered Sheet 2)

A pressure-sensitive adhesive layer-forming layered sheet 2 comprisingPET film/UV-crosslinkable pressure-sensitive adhesive layer (B-1;thickness: 65 μm)/PET film was produced in a similar manner to thepressure-sensitive adhesive material layer-forming layered sheet 1,except that, in the pressure-sensitive adhesive material layer-forminglayered sheet 1, the coating and the forming were such that thethickness of the pressure-sensitive adherent layer (B-1) was 65 μm.

(Pressure-Sensitive Adhesive Layer-Forming Layered Sheet 2′)

A pressure-sensitive adhesive layer-forming layered sheet 2′ comprisingPET film/heat-crosslinkable pressure-sensitive adhesive layer (B-1′;thickness: 65 μm)/PET film was produced in a similar manner to thepressure-sensitive adhesive layer-forming layered sheet 2 describedabove, except that the coating substrate was changed to arelease-treated polyethylene terephthalate film (manufactured byMitsubishi Plastics, “MRF50”; thickness: 50 μm).

Example 1 Production of Adhesive Sheet 1

The PET film on both sides of the intermediate resin layer (A-1) in theintermediate resin layer-forming layered sheet 1 were sequentiallypeeled and removed, while a PET film on one side of thepressure-sensitive adhesive layers (B-1) and (B-1′) in thepressure-sensitive adhesive layer-forming layered sheets 1 and 1′ werepeeled and the exposed adhesive surfaces were sequentially bonded toboth surfaces of (A-1) with a laminator to produce a multilayer adhesivesheet comprising (B-1)/(A-1)/(B-1′).

UV light was irradiated through the polyethylene terephthalate filmsremaining on the surfaces of (B-1) and (B-1′) with a high-pressuremercury lamp so that the integrated amount of light at 365 nm was 1,000mJ, UV-crosslinking (B-1), (A-1) and (B-1′) to produce a pre-secondarycuring transparent double-sided adhesive sheet 1 (total thickness: 180μm).

<Example 2 Production of Adhesive Sheet 2

The intermediate resin layer composition (A-2) and thepressure-sensitive adhesive composition (B-1) were co-extruded so as tobe pressure-sensitive adhesive layer (B-1)/intermediate resin layer(A-2)/pressure-sensitive adhesive layer (B-1), coated and formed so asto have pressure-sensitive adhesive layer (B-1)/intermediate resin layer(A-2)/pressure-sensitive adhesive layer (B-1)=40/100/40 μm over arelease-treated polyethylene terephthalate film (Mitsubishi resinous“MRA75”; thickness: 75 μm) and covered with a release-treatedpolyethylene terephthalate film (manufactured by Mitsubishi Plastics,“MRA50”; thickness: 50 μm) to form a multilayer sheet comprising PETfilm/(B-1)/(A-2)/(B-1)/PET film.

UV light was irradiated from one polyethylene terephthalate film sidewith a high-pressure mercury lamp so that the integrated amount of lightwas 1,000 mJ, UV-crosslinking (B-1), (A-2) and (B-1) to produce apre-secondary curing transparent double-sided adhesive sheet 2 (totalthickness: 180 μm).

Example 3 Production of Adhesive Sheet 3

Mixed and added into 1 kg of the acrylic acid ester copolymer A were 50g of nonane diol diacrylate as crosslinking agent and10 g of 4-methylbenzophenone as photo-polymerization initiator to prepare apressure-sensitive adhesive composition (B-2).

This adhesive composition was heat-melted, coated with an applicator soas to become 170 μm in thickness to form a film over a release-treatedpolyethylene terephthalate film (manufactured by Mitsubishi Plastics,“MRF75”; thickness: 75 μm) and covered with a release-treatedpolyethylene terephthalate film (manufactured by Mitsubishi Plastics,“MRA50”; thickness: 50 μm) to form a sheet comprising PETfilm/pressure-sensitive adhesive layer (B-2)/PET film.

UV light was irradiated from one polyethylene terephthalate film sidewith a high-pressure mercury lamp so that the integrated amount of lightwas 1,000 mJ, UV-crosslinking (B-2) to produce a pre-secondary curingtransparent double-sided adhesive sheet 3 (total thickness: 170 μm).

Example 4 Production of Adhesive Sheet 4

A pre-secondary curing transparent double-sided adhesive sheet 1 (totalthickness: 260 μm) comprising the layer constitution (B-1)/(A-2)/(B-1′)was produced in a similar manner to Example 1, except that theintermediate resin layer-forming layered sheet 2 (A-2) was used as theintermediate resin layer and the pressure-sensitive adhesivelayer-forming layered sheets 2 and 2′ were used as pressure-sensitiveadhesive layers.

Comparative Example 1 Production of Adhesive Sheet 5

An acrylic acid ester copolymer (Mw=1,000,000; theoretical Tg: −61 ° C.)was prepared, comprising 48 parts by mass of 2-ethylhexyl acrylate, 50parts by mass of 2-methoxyethyl acrylate (homopolymer Tg: −50° C.) and 2parts by mass of 4-hydroxybutyl acrylate (homopolymer Tg: −80° C.),which were random-copolymerized.

Added with respect to 1 kg of this acrylic acid ester copolymer was 0.2parts by mass in solid contents of adduct-type hexamethylenediisocyanate (manufactured by Asahi Kasei, DURANATE P301-75E) ascrosslinking agent to prepare an adhesive material composition.

The adhesive composition was coated, formed and dried over arelease-treated polyethylene terephthalate film (Mitsubishi Plastics,“MRA75”; thickness: 75 μm) then covered with a release-treatedpolyethylene terephthalate film (Mitsubishi Plastics, “MRF50”;thickness: 50 μm), and cured for one week under the conditions of 25° C.in temperature and 50% humidity for heat-crosslinking to produce atransparent double-sided adhesive sheet 5 (total thickness: 150 μm).

Comparative Example 2 Production of Adhesive Sheet 6

A post-secondary curing transparent double-sided adhesive sheet 6 (totalthickness: 180 μm) was produced, comprising the pre-secondary curingtransparent double-sided adhesive sheet 1 (total thickness: 180 μm)additionally irradiated with a high-pressure mercury lamp so that afurther 1,000 mJ of UV light at 365 nm wavelength reached the adhesivelayer, causing secondary-curing of (B-1), (A-1) and (B-1′).

<Evaluation>

The adhesive sheets 1 to 6 described above were evaluated as follows:

(Indentation Hardness Measurement)

For the adhesive sheets 1 to 6 described above, release films werepeeled, and the exposed adhesive surfaces were sequentially overlappedto layer several adhesive sheets so that the total thickness was withinthe range of 5 mm to 7 mm. This allows the influence of the hardness ofthe stage where the measurement sample is placed to be reduced, allowingthe indentation hardness specific to the material to be compared andmeasured.

Then, the indentor of an Asker C2L durometer was pressed at a speed of 3mm/minute vertically downward with a 1 kg load against the exposedadhesive surface of the layered adhesive sheet to measure the C2 Askerhardness (a).

In addition, for the adhesive sheets 1 to 4, ultraviolet irradiation wascarried out from one of the release film sides while checking theintegrated amount of light with an actinometer (UNIMETER UIT150 TYPEUIT-150A manufactured by Ushio Inc.) so that 2,000 mJ/cm² of UV light at365 nm wavelength reached the adhesive sheet to produce adhesive sheetscorresponding to post-secondary curing transparent double-sided adhesivesheets.

By a similar method to the above description, layering was carried outso that the total thickness was within the range of 5 mm to 7 mm and theindentor of the Asker C2L durometer was pressed at a speed of 3mm/minute vertically downward with a 1 kg load against the exposedadhesive surface to measure the C2 Asker hardness (b).

(Processability)

With the release films still layered, the adhesive sheets 1 to 6described above were cut using a Thompson die-cutter into 100 pieceswith a 50 mm×80 mm Thompson blade, and the shapes of the edges of thecut sheets were visually observed.

Then, the assessment was “x (cross) (fail)” for those that had 10 piecesor more with glue flashes at the sheet edges and “o (circle) (pass)” forthose with less than 10 pieces.

(Unevenness Followability)

A polarizer (manufactured by Nitto Denko Corp.; NWF-KDSEGHC-ST22) waspre-bonded to one of the sides above a glass plate (53×83 mm×t 0.5 mm)over the same dimensions to produce a plate serving as an adherend fortest purposes, 0.03 mg of glass beads (approximately 200 particles) witha median particle size of 50 μm were scattered as pseudo-bubblingorigins on the polarizer surface, which served as a substrate foranti-bubble reliability tests. A first release film of the adhesivesheets 1 to 6 cut in the processability evaluation was peeled and theexposed adhesive surface was adhered onto the substrate with a handroller. Next, the remaining release film was peeled, a soda lime glass(53×83 mm×t 0.5 mm) was press-bonded to the exposed adhesive surfaceunder reduced pressure (absolute pressure: 5 kPa), and then, autoclavetreatment (50° C., 0.2 MPa, 20 minutes) was carried out for finishingbonding to produce a laminate (sample).

The produced laminates were visually observed, and the assessment was “o(circle) (pass)” for those in which the surface of glass beads scatteredover the substrate were wetted by the adhesive sheet and the beadsperiphery was buried in the adhesive layer without a gap, and “x (cross)(fail)” for those in which there was lifting in the glass beadsperiphery.

(Anti-Bubble Reliability)

For the adhesive sheets 5 to 6, the laminates (samples) produced in theunevenness followability test described above served as samples for theanti-bubble reliability test.

Meanwhile, for the adhesive sheets 1 to 4, ultraviolet was irradiatedagainst the laminates (samples) produced in the unevenness followabilitytest described above through the soda lime glass with a high-pressuremercury lamp so that 2,000 mJ/cm² of UV light at 365 nm wavelengthreached the adhesive sheets 1 to 4 to produce samples for theanti-bubble reliability test.

Each of these samples were left alone in a normal state (temperature:23° C.; humidity: 50%) for one day, then, cured for 6 hours in athermo-humidistat at 85° C. in temperature and 25% humidity, and theexternal appearance after curing was visually observed.

The assessment was “o (circle) (pass)” for those in which no new liftingoccurred from the glass beads periphery and “x (cross) (fail)” for thosein which bubbling or lifting occurred due to curing.

TABLE 1 Comparative Example Example 1 2 3 4 1 2 Adhesive sheet  1  2  3 4  6  7 C2 Asker hardness (a) 29 32 35 45 15 35 C2 Asker hardness (b)37 37 40 48 — — Processability ◯ ◯ ◯ ◯ X ◯ Unevenness followability ◯ ◯◯ ◯ ◯ X Anti-bubbling reliability ◯ ◯ ◯ ◯ X ◯ Overall evaluation ◯ ◯ ◯ ◯X X

Examples 1 to 4 are all pre-secondary curing transparent double-sidedadhesive sheets in a state where UV-reactivity was left by forming anadhesive composition into the form of a monolayer or multilayer sheetand primary curing this by UV-crosslinking.

If two image display device-constituting members were layered throughthe pre-secondary curing transparent double-sided adhesive sheetdescribed above, UV light was irradiated from the image displaydevice-constituting member side, and the pre-secondary curingtransparent double-sided adhesive sheet was UV-crosslinked to besecondary cured through this member, firmly crosslinking was found to bepossible, allowing a transparent double-sided adhesive sheet aftersecondary curing to be obtained with a high indentation hardness.

In addition, in the relationship between the indentation hardness (C2Asker hardness) (a) of the pre-secondary curing transparent double-sidedadhesive sheet and the indentation hardness (C2 Asker hardness) (b) ofthe transparent double-sided adhesive sheet after secondary curing, itwas found that, unevenness followability and anti-bubble reliabilitycould be improved by controlling the relationship to be (b)−(a)≧2.

1. A method for producing a laminate, the method comprising: forming an adhesive composition into a form of a monolayer or a multilayer sheet; primary-curing the monolayer or multilayer sheet by UV-crosslinking to obtain a pre-secondary curing transparent double-sided pressure-sensitive adhesive sheet; layering two image display device-constituting members via the pre-secondary curing transparent double-sided adhesive sheet and then irradiating ultraviolet light from a side of an image display device-constituting member, thereby UV-crosslinking and secondary-curing the pre-secondary curing transparent double-sided adhesive sheet through the member.
 2. The method of claim 1, wherein the forming the adhesive composition comprises forming the adhesive composition into a sheet above a transparent release sheet and the primary-curing comprises irradiating UV light through the transparent release sheet to UV-crosslink the adhesive composition.
 3. The method of claim 1, wherein the pre-secondary curing transparent double-sided adhesive sheet has a wavelength absorption for starting a UV-crosslinking reaction of from 315 nm to 400 nm.
 4. The method of claim 1, wherein an indentation hardness (b) of the transparent double-sided adhesive sheet after the secondary curing is 33≦(b)≦80.
 5. The method of claim 1, wherein an indentation hardness (a) of the pre-secondary curing transparent double-sided adhesive sheet after the primary-curing and an indentation hardness (b) of the transparent double-sided adhesive sheet after secondary curing satisfy a relation (b)-(a)≧2.
 6. The method of claim 1, wherein, the irradiating ultraviolet light from a side of an image display device-constituting member comprises irradiating ultraviolet light through a member with a light transmittance of 20% or greater in a wavelength range of 315 to 400 nm.
 7. The method of claim 1, wherein, the image display device-constituting members are each laminates comprising a touch panel, an image display panel, a surface protection panel, a polarization film, or any combination thereof.
 8. An image display device comprising a laminate obtained by a process comprising the method of claim
 1. 9. The method of claim 1, wherein the adhesive composition comprises a (meth)acrylic acid ester series polymer.
 10. The method of claim 1, wherein the adhesive composition is a solvent-free system that does not contain a solvent and is hot-melt formable.
 11. The method of claim 1, wherein a thickness of the monolayer or multilayer sheet is from 50 μm to 1 mm.
 12. The method of claim 1, wherein the monolayer or multilayer sheet is a multilayer sheet comprising outermost layers and an intermediate layer, and a ratio of a thickness of each of the outermost layers to a thickness of the intermediate layer is from 1:1 to 1:20.
 13. The method of claim 1, wherein an indentation hardness of the pre-secondary curing transparent double-sided adhesive sheet after the primary-curing is from 10 to
 50. 14. The method of claim 1, wherein the forming an adhesive composition comprises forming the adhesive composition into a monolayer sheet.
 15. The method of claim 1, wherein the forming an adhesive composition comprises forming the adhesive composition into a multilayer sheet. 